1. Field of the Invention
The invention relates to a refractive projection objective for projecting a pattern arranged in an object plane of the projection objective into an image plane of the projection objective with the aid of an immersion medium which is arranged between a last optical element of the projection objective and the image plane.
2. Description of the Related Art
Photo-lithographic projection objectives have been in use for several decades for producing semiconductor components and other finely structured structural elements. They serve the purpose of projecting patterns of photomasks or reticles, which are also denoted below as masks or reticles, onto an object coated with a photosensitive layer with very high resolution on a reducing scale.
Three developments running in parallel chiefly contribute to the production of every finer structures of the order of magnitude of 100 nm or below. Firstly, an attempt is being made to increase the image-side numerical aperture (NA) of the projection objective beyond the currently customary values into the region of NA=0.8 or above. Secondly, ever shorter wavelengths of ultraviolet light are being used, preferably wavelengths of less than 260 nm, for example 248 nm, 193 nm, 157 nm or below. Finally, still other measures are being used to increase resolution, for example phase-shifting masks and/or oblique illumination.
In addition, there are already approaches to improving the achievable resolution by introducing an immersion medium of high refractive index into the space between the last optical element of the projection objective and the substrate. This technique is denoted here as immersion lithography. Introducing the immersion medium yields an effective wavelength ofλeff=λ0/n,λ0 being the vacuum operating wavelength and n the refractive index of the immersion medium. This yields a resolution ofR=k1(λeff/NA0)and a depth of focus (DOF) ofDOF=±k2(λeff/NA02),NA0=sin Θ0 being the “dry” numerical aperture, and Θ0 being half the aperture angle of the objective. The empirical constants k1 and k2 depend on the process.
The theoretical advantages of immersion lithography reside in the reduction of the effective operating wavelength and the resolution improved thereby. This can be achieved in conjunction with an unchanged vacuum wavelength, and so established techniques for producing light for selecting optical materials, for coating technology etc. can be adopted largely without change for the appropriate wavelength. However, measures are required for providing projection objectives with very high numerical apertures in the region of NA=1 or above. Furthermore, suitable immersion media must be available.
The article entitled “Immersion Lithography at 157 nm” by M. Switkes and M. Rothschild, J. Vac. Sci. Technol. Vol. 19 (6), November/December 2001, pages 1 ff. presents immersion fluids based on perfluoropolyethers (PFPE) which are sufficiently transparent for a working wavelength of 157 nm and are compatible with some photo-resist materials currently being used in micro-lithography. One tested immersion fluid has a refractive index of n=1.37 at 157 nm. The publication also describes a lens-free optical system, operating with calcium fluoride elements and silicon mirrors, for immersion interference lithography, which is intended to permit the projection of 60 nm structures and below in conjunction with a numerical aperture of NA=0.86. The optical system may not be suitable for use in the series production of semiconductors or the like.
U.S. patent specification U.S. Pat. No. 5,610,683 (corresponding to EP 0 605 103) describes a projection exposure machine, provided for immersion lithography, having devices for introducing immersion fluid between the projection objective and the substrate. No design is specified for the optical projection system.
U.S. patent specification U.S. Pat. No. 5,900,354 proposes using a super-critical fluid, for example xenon gas, as immersion medium in immersion lithography. No design is shown for a suitable projection objective.